1. Field of the Invention
The present invention is directed to waterborne coating compositions containing an aqueous polymer dispersion and a non-polymeric material containing an acetoacetyl group which serves as a coalescing agent and promotes adhesion of the film formed from the coating composition to a substrate.
2. Background of the Invention
Waterborne coatings comprise particles of polymeric materials dispersed in an aqueous medium. When applied to a substrate and dried, the particles coalesce to form films which protect the substrate. Certain additives such as coalescing agents or plasticizers are frequently used to promote coalescence of the latex particles in the coating during film formation so that the resulting film has a smooth glossy appearance and completely covers and protects the substrate. Other additives are used to improve adhesion of the coating film especially on metal substrates.
U.S. Pat. Nos. 4,296,226 and 4,421,889 teach that polymers having acetoacetyl pendant groups have improved adhesion to smooth non-absorbent surfaces. Further, U.S. Pat. Nos. 5,055,511 and 5,120,607 describe the use of acetoacetylated acrylic polymers in mastic caulking compositions. Other patents, such as U.S. Pat. No. 5,002,998, EP 390,370, EP 262,720 and EP 326,723, and Japanese Kokai Hei 4 (1992)-164873 describe the use of polymers containing acetoacetoxy groups bound to the polymeric structure. U.S. Pat. No. 5,349,026 describes the use of a polymer containing an acetoacetate group in a coating or impregnating composition. The polymer functions to coalesce and crosslink an emulsion polymer. U.S. Pat. No. 5,494,975 describes preparation of polymers containing functional acetoacetate groups which are then reacted with a functional amine to form an enamine.
In all of the above references, the acetoacetoxy group is chemically bound to the polymer which forms the coating. This severely limits the chemical nature of the polymeric coating, since the polymer must either be based upon an acetoacetylated copolymerizable monomer or it must contain pendant groups, such as hydroxy or amine, which are capable of reacting with an acetoaceylating agent.
Japanese Patent 3,290,486 describes a process for promoting adhesion to a tin-lead alloy plated steel sheet which involves immersing the sheet in an aqueous acetoacetic acid ester prior to painting.